Minimally invasive medical devices such as stents, grafts, and balloon catheters, are used for a number of medical purposes including the treatment of vascular disease, reinforcement of recently re-enlarged lumens, and the replacement of ruptured vessels. It is often beneficial to incorporate in such medical devices, therapeutic agents, which are often contained within a coating applied to the surface of the medical device, to provide desired therapeutic properties and effects. For example, it is useful to apply a coating containing therapeutic agents to medical devices to provide for the localized delivery of therapeutic agents to target locations within the body, such as an occluded body lumen. Compared to systemic drug administration, such localized drug delivery minimizes unwanted effects on parts of the body which are not to be treated and allows for the delivery of higher amounts of therapeutic agent to the afflicted part of the body. Coatings containing therapeutic agents may also provide for controlled release, which includes long-term or sustained release, of the therapeutic agent.
Conventionally, coatings have been applied to medical devices by processes such as dipping, spraying, vapor deposition, plasma polymerization, and electro-deposition. Although these processes have been used to produce satisfactory coatings, there are numerous drawbacks associated with these processes. For example, with current spraying processes, it is difficult to achieve a narrow weight distribution of the therapeutic agent on the medical device. Furthermore, the evaporation of solvents used in the coatings, the complex geometries of many medical devices, and the low amounts of therapeutic agents applied to the coatings contribute to the difficulty in measuring the amount of therapeutic agent actually disposed on the medical device. In addition, there is no direct feedback loop associated with these medical devices to ascertain the actual amount of therapeutic agent disposed on the medical device and therefore it is often necessary to maintain processing parameters as constant as possible. However, due to the large number of processing parameters associated with coating a medical device, there is still a significant amount of variation in the amount of drug disposed on the medical device.
In addition to the difficulties associated with achieving a narrow weight distribution of a therapeutic agent on a medical device, current coating procedures are designed to result in a uniform application of therapeutic agent across the medical device, which is suitable and often desired for many current medical devices. Such a coating process, however, does not allow for differentiation in drug amount across the medical device. Therefore, there is a need in the art for a coating process that allows for a broader weight distribution on the medical device and that allows for non-uniform application of the therapeutic agent on the medical device.